The regulated expression of specific Mtb genes is likely required for the establishment of tuberculosis infections. One family of gene regulators present in M. tuberculosis is the two component regulators. Two component regulatory systems sense and respond to environmental stimuli. tcrA (for two component regulator) is a two component regulatory gene found in M.tb. tcrA is present in M. tuberculosis isolates and in M. bovis isolates, but is absent in mycobacterial strains which belong to the four nontuberculosis groups of mycobacteria. tcrA has a regulatory effect on Mtb protein expression as evidenced by multiple changes in the synthesis of Mtb proteins in a tcrA mutant compared to Mtb Erdman. The objective of this proposal is to evaluate the role of the tcrA regulatory system in the production of tuberculosis infections. A tcrA mutant of Mtb Erdman will be used to assess the importance of the tcrA gene in Mtb growth in cultured macrophages. The sensitivity of the tcrA mutant to toxic macrophage compounds will be evaluated and the morphologic appearance of macrophages infected with the tcrA mutant analyzed. Mice will be infected with the tcrA mutant using aerosol inoculation and the resulting infection compared with Mtb Erdman infections. tcrA regulated genes will be cloned, mutants in each gene will be constructed and each mutant evaluated for growth in cultured macrophages. The environmental conditions which activate the tcrA regulon will also be identified. These studies will be valuable in identifying MTb genes which contribute to Mtb pathogenicity and which may serve as targets for anti-tuberculosis treatment either in vaccines or for anti-mycobacterial drugs.